Dicarboxylic aromatic acids such as phthalic acid, isophthalic acid and terephthalic acid may be prepared by an oxidation process in which mixed alkyl aromatics or particular xylenes are subjected to an oxidation reaction with heavy metal salts and bromine acting as catalysts. For example, paraxylene may be oxidized to terephthalic acid in the liquid phase with a catalyst consisting of cobalt acetate and hydrogen bromide dissolved in acetic acid. An oxygen containing gas such as air is injected into the reactor which is maintained at an elevated temperature of around 200.degree. C. and superatmospheric pressures of about 10 atomspheres. The amount of air which is injected into the reactor is sufficient to maintain an excess of oxygen in the off-gas over the stoichiometric requirement. The liquid which is drawn from the reactor will contain about 25 to about 35 percent solids. These solids contain the desired terephthalic acid as well as a contaminant or impurity in the form of paracarboxybenzaldehyde, an intermediate in the formation of terephthalic acid. As the reaction proceeds and the terephthalic acid crystallizes out of the reaction medium, it entrains with it the solid paracarboxybenzaldehyde in the form of mixed crystals, thereby retaining the further oxidation of impurity difficult to achieve.
In the past, the working up and purification of terephthalic acid has presented considerable problems inasmuch as the aforesaid paracarboxybenzaldehyde is a difficult compound to remove by the usual purification methods. In view of the greater solubility of paracarboxybenzaldehyde in acetic acid relative to terephthalic acid, the amount of impurity in the terephthalic acid will be greatly reduced but will still be in excess of the maximum amount allowable in the acid. Therefore, it is necessary to utilize a relatively complex purification system including various steps such as oxidation, neutralization, recycling, washing, distillation, etc.
Several U.S. patents have disclosed methods for purifying acids, and particularly aromatic dicarboxylic acids. For example, U.S. Pat. No. 3,546,285 discloses a method for purifying aromatic dicarboxylic acids such as terephthalic acid by catalytically hydrogenating the acid which is dissolved in a solvent at an elevated temperature. The catalytic hydrogenation compound which is employed in this process comprises a noble or other metal of Group VIII of the Periodic Table such as platinum, palladium, nickel, etc. Likewise, U.S. Pat. No. 3,151,154 is drawn to a method for decolorizing phthalic acids or salts thereof which have been prepared by the nitric acid oxidation of diloweralkyl benzenes and loweralkyl benzoic acids by reducing the nitro compounds in a liquid polar solvent in the presence of a solid Group VIII metal hydrogenation catalyst and thereafter acidifying the resulting reduced product.
U.S. Pat. No. 3,607,921 utilizes a process for purifying terephthalic acid by contact, in the presence of carbon monoxide, with solid particles of an adsorptive agent which possesses substantial carbon monoxide sorption capacity, while U.S. Pat. No. 3,456,001 purifies these acids by utilizing a support Group VIII noble metal catalyst such as platinum, palladium, ruthenium, rhodium, iridium, and osmium.
Yet another patent which teaches a purification process is U.S. Pat. No. 3,522,298. This patent teaches the purification of terephthalic acid by contacting a vaporous mixture and a gaseous medium with a solid material comprising a Group VIII metal, said gaseous medium comprising either hydrogen or oxygen.
As will hereafter be shown in greater detail, it has now been discovered that it is possible to remove contaminants or impurities from dicarboxylic aromatic acids in a relatively inexpensive and simple manner, said method involving the catalytic decarbonylation of contaminants utilizing a catalyst comprising a metallic rhenium containing compound while effecting the reaction in a liquid phase without requiring the presence of either hydrogen or oxygen.